As a transmission system for a vehicle, a combination of a fluid transmission such as a torque converter or fluid coupling, and a continuously variable transmission which transmits the output torque of the fluid transmission to a vehicle drive shaft after changing the speed in a stepless manner, is disclosed for example in Tokkai Sho 61-105353 published by the Japanese Patent Office in 1986. This system comprises a controller which sets a target speed change ratio based on vehicle running conditions such as engine load represented by a throttle opening or vehicle travel speed, and controls the speed change ratio of the continuously variable transmission so that it coincides with the set target speed change ratio.
In addition to this automatic speed change mode referred to as D range, a manual mode known as L range is also provided wherein the driver can manually select a specific speed change ratio, e.g. a speed change ratio corresponding to the low gear of a conventional manual transmission system.
By providing this manual speed change mode, the driver can specify the low speed range when for example, the vehicle is running with a small speed change ratio in the D range, and the vehicle can therefore be driven under conditions desired by the driver which are outside the preset automatic speed change pattern. This option makes it possible to improve vehicle running performance under various non-standard running conditions.
However, in such a manual speed change mode, the number of speed change ratios that can be selected is limited. Also selection of speed change ratio in the manual mode is performed by an operation similar to using a shift lever which was difficult for a driver accustomed to the automatic speed change mode.